This invention relates to thermoplastic sheet strip heaters.
Heaters have heretofore been devised for heating thermoplastic materials and products in order to soften the material and render it readily workable. U.S. Pat. No. 3,816,705, for example, discloses a heater specially adapted to heat thermoplastic eyeglass frames in order to soften them to permit insertion of lenses or to bend them to fit the face and head of a user. Strip heaters have also been devised for heating selected strip portions of thermoplastic materials in sheet form in order to render them bendable along the heated strip without having to heat other portions that are not to be bent or worked. U.S. Pat. No. 4,034,205 provides one example of such prior art strip heaters. The American Cynamid Company, Plastics Division, of Wallingford, Connecticut discloses a flat strip heater in their Acrylite Acrylic Sheet Fabrication Manual (1970) as does the Rohn and Haas Company of Philadelphia, Pennsylvannia in their 1971 handbook titled Fabrication of Plexiglas. Rohm and Haas also illustrates a strip heater in their 1974 brochure titled Do It Yourself With Plexiglas, Third Edition. The Briscoe Manufacturing Company of Columbus, Ohio further offers a strip heater referred to as the Briskeat RH-36 Heating Element for use in working with Rohm and Haas' Plexiglas acrylic plastic.
Strip heaters are relatively basic in construction and unrefined in operating efficiency. For example, the prior strip heaters appear to have had problems maintaining consistent and controllable heat development within the housing, especially in regards to those heaters which attempt to vary the effective heating width of the heater. Furthermore, side walls of the heater housings of prior heaters have had a tendency to warp in actual use as a result of extreme amounts of heat. This warping appears to be a source of non-uniform heating width along the length of the flue. Prior art heaters also generally create sharply defined lines, known in the trade as "mark-offs", between adjacent-heated and non-heated areas of the plastic sheet. This "mark-off" is undesirable and is often due to restricted lateral heat dissipation resulting from the unsatisfactory design of prior art heaters. Strip heaters have not, in general, provided the user with the ability to readily locate the to-be-heated portion of the plastic over the heater opening, and those heaters which have provided such capability have been large and cumbersome. Strip heaters have also failed, in the past, to have good capability for heating multiple, spaced strips of sheet material simultaneously and without having to relocate the work piece. Strip heaters of the double type which are adapted to heat both sides of a sheet strip simultaneously have not been provided with means for successfully assuring a flush placement of both heater housings against the work piece. Nor have they been provided with means for automatically timing heating operations.